In order to determine the molecular nature of NK cell recognition of target cells, K562, plasma membranes were prepared from the human NK cell target plasma membrane proteins were solubilized with the detergent CHAPS, and artificial membranes bearing the membrane protein formed from exogeneous lipids after detergent removal by dialysis. These artificial membranes were found to be capable of inhibiting "congugates", i.e., killer-target pairs identifiable in the microscope, between human large-granular lymphocytes and K562 cells. The specificity of this inhibition reflects the specificity of NK killing. Solubilized membrane proteins were subjected to various treatments and fractionation procedures. It was found that the active material was heat sensitive, Typsin sensitive and could be bound to and eluted from a ConA Sephamore column.